Aviation safety investigations & reports

Saab Aircraft AB SF-340B, VH-OLM

Summary

On the evening of 28 June 2002, a Saab 340B, VH-OLM, was being
operated on a regular public transport service, from Sydney to
Bathurst, NSW. The pilot in command (PIC), the flying pilot, had
commenced a descent from 12,000 ft for a Katoomba-Bathurst Global
Positioning System (GPS) arrival and subsequent landing on runway
17 at Bathurst. The PIC reported that as the aircraft descended to
the minimum descent altitude (MDA), visibility alternated between
visual and instrument flight conditions. During the descent, the
PIC had retarded the power to about 17 per cent and slowed the
aircraft to about 135 kts in preparation for a Category B circling
approach.

The copilot, non-flying pilot, reported that during the descent
the engine anti-ice was on, but not the propeller de-ice, nor had
the airframe boot de-ice system been activated. The PIC reported
that during descent, they entered cloud a number of times and noted
ice accretion on the windshield wiper. The flight crew reported
that they did not observe any wing ice during the descent.

At the MDA (3,810 ft), the aircraft's Flight Guidance and
Autopilot System (autopilot) captured the altitude and, as the
airspeed was decreasing due to the reduced power setting, commanded
the trim system to progressively raise the nose of the aircraft to
maintain the MDA. The PIC commanded the autopilot to roll the
aircraft to the right to begin tracking downwind for runway 17. At
about this time, the copilot observed that the airspeed was
decreasing and called 'speed'. As the PIC applied power to
compensate for the decreasing airspeed, the aircraft rolled to the
left and pitched down without warning. During the recovery from the
steep pitch and bank angles, the aircraft rolled to the right and
descended to 112 ft AGL. The PIC regained control of the aircraft
and climbed it to the missed approach altitude and carried out an
uneventful landing.

The aircraft's aerodynamic stall warning systems of stick
shaker, audible alarm, visual warnings and stick pusher, did not
activate during the initial roll to the left. However, theautopilot
disconnected during the subsequent roll to the right, due to
activation of the stall warning.

The investigation determined that following capture of the MDA
by the autopilot, the aircraft speed continued to decrease due to
the reduced power setting. As a consequence,the aircraft stalled.
However, this occurred prior to the stall warning system operating
due to the likely presence of airframe ice that had accumulated
during the descent.

The investigation found that it is possible for the aircraft to
stall prior to the activation of the stall warning system if the
aircraft has accumulated ice on the wings.

The investigation, classed as a serious incident, identified a
number of other occurrences involving Saab 340 aircraft stalling
where little to no stall warning had been provided to flight crew
while operating in icing conditions. This included a Saab operated
by an Australian operator, which resulted in a number of ATSB
recommendations being issued in that investigation, not all of
which were accepted and acted upon. Some of those recommendations
have been re-issued.